Chromogenic MoO₃ thin films: thermo-, photo-, and electrochromic response to working pressure variation in rf reactive magnetron sputtering

The thermochromic, photochromic, and electrochromic properties of molybdenum trioxide (MoO₃) thin films were studied. MoO₃ thin films were deposited by rf reactive magnetron sputtering and the influence of deposition parameters, i.e. O₂/Ar gas ratio and working pressure, on the chromogenic properties was investigated. Thermochromism was induced by annealing the samples in either air or argon in the range 23–300 °C for 2 h. We found that the highest response was obtained for samples grown at 5.3 × 10⁻¹ Pa, although films annealed in air showed a maximum coloration around 250 °C that became bleached above this temperature. As for the annealing in argon, the thermochromic effect increased even at 300 °C. By exposing samples to UV irradiation in air, photochromism could be induced for different intervals ranging from 0 to 3 h. The highest photochromic response was obtained for samples deposited at 1.3 Pa. Cyclic voltammetry for 20 cycles in a 1 M LiClO₄ in propylene carbonate solution, inside a glovebox filled with argon, was used to evaluate the electrochromic response. Samples that showed optimum electrochromic response were deposited at 1.6 Pa. These results are explained in terms of the optical, structural, surface chemical composition, and vibrational modes.